Variability in carbon exchange and light utilization among boreal forest stands: implications for remote sensing of net primary production

Variability in carbon exchange, net primary production (NPP), and light-use efficiency were explored for 63 boreal forest stands in northeastern Minnesota using an ecophysiological model. The model was initialized with extensive field measurements of Populus tremuloides Michx. and Picea mariana (Mill.) BSP stand properties. The results showed that the proportion of total carbon assimilation expended in autotrophic respiration (i.e., the respiration to assimilation ratio, R/A) was significantly different for the two tree species and this explained much of the variability in the amount of net production per unit absorbed photosynthetically active radiation (APAR), referred to as PAR utilization (epsilon(n)). This is the first known study to directly link variability in respiratory costs to epsilon(n). Total assimilation per unit APAR (epsilon(g)) was much less variable than epsilon(n) and was not significantly different between species. Greater stomatal control on some moisture stressed sites accounted for most of the variability in epsilon(g). The lack of a simple relationship between light harvesting and net carbon gain indicates that estimation of net primary production with satellite remote sensing requires additional information on respiration costs; however, evidence for convergence in epsilon(g) can be used to simplify the remote sensing of gross primary production over large areas.